This is great. I really think that having a theme to iterate on is the best way of revisiting coding/math skills and learning new ones. And building things from scratch is so much more rewarding than snapping parts together. My free time right now is trying to rewrite 90s screensavers down to the rasterizer level, without consulting anything for how to do stuff like antialiasing.

One little note, just checking out your loan calculator: On my phone, the total interest div is cut off at the decimal point, and the total payment overflows the nice round rect holder. You could dial in the font size scaling with a bit of custom Javascript there. They stand out, too, because they're bottom line figures.